1. Field of the Invention
The present invention pertains to methods and systems for scanning a beam of charged particles, such as an electron beam, across a conveyor path and, more particularly, to such methods and systems wherein the beam is moved incrementally across the conveyor path and the radiation dose at each incremental position is controlled only by movement of the beam.
2. Discussion of the Prior Art
Objects moving along a conveyor path are conventionally irradiated with beams of charged particles to produce desirable effects, such as altering characteristics by cross-linking, sterilizing, and facilitating curing among others. In order to irradiate all surfaces of objects moving along a conveyor path, the beam of charged particles is normally scanned across the conveyor path by means of an electrically energized scanning magnet for sweeping the beam in a scanning horn or, in some cases, by means of moving magnets or selectively controlled magnets. U.S. Pat. No. 2,887,583 to Emanuelson, U.S. Pat. No. 2,897,365 to Dewey II et al, U.S. Pat. No. 3,246,147 to Skala, U.S. Pat. No. 3,193,717 to Nunam, U.S. Pat. No. 3,687,716 to Steigerwald and U.S. Pat. No. 3,442,017 to Uehara et al are examplary of such prior art scanning systems.
In many applications, the radiation dose required at different locations on the objects varies in accordance with the configuration of the object or a specific pattern of irradiation required. Accordingly, it has been the practice in the prior art to vary the intensity of the beam, that is, vary beam voltage or current, during scanning, such as by means of a beam control device as described in the Steigerwald U.S. Pat. No. 3,687,716, in order to vary the irradiation dosage to which portions of an object to be irradiated are subjected. Although such systems are completely acceptable in some applications, they do not lend themselves to systems in which dose rate must be maintained at a precise level or below a specific level and/or systems in which the beam must impinge upon precise discrete and often discontinuous regions of the object.
The present invention has a further object in that a scanning system for a beam of charged particles controls the beam to move it incrementally across a conveyor path while maintaining the beam at each incremental position for preselected times to control irradiation dosage.
An additional object of the present invention is to utilize a plurality of sequentially energized controllable deflection magnets to scan, spread, distribute, disperse or deflect a small diameter, essentially parallel beam of charged particles across a conveyor path along which objects or material to be irradiated are moved in order to produce a uniform or controlled non-uniform irradiation dosage to a continuous or interrupted path across the full width of the conveyor.
The present invention has another object in that the energization time of each of a plurality of controllable deflection magnets can be varied between zero and prescribed discrete time intervals to control the irradiation pattern and dose on an object to be irradiated with smooth gradations in dose distribution obtained by increasing or decreasing the pulse widths of successive pulses supplied to the deflection magnets.
Yet a further object of the present invention is to control the scanning of a beam of charged particles across a conveyor path to produce selective irradiation pattern and/or dosage by deflecting the beam through a plurality of selected positions across the conveyor path and maintaining the beam at each of the selected positions for a preselected time in accordance with the irradiation dosage required at each of the selected positions.
The present invention has as an additional object, the control of the irradiation dosage at various positions across a conveyor path by deflecting the beam incrementally from position to position and controlling the length of time the beam remains at each position, whereby does variation is obtained with the use of a constant beam current load on the accelerator thus stabilizing the high voltage potential, i.e. beam energy.
Additional objects of the present invention over the prior art are to provide a method and system having increased versatility and more adaptability to a variety of irradiation processes than conventional beam scanning methods and systems, utilizing in one embodiment a plurality of sequentially operated controllable deflection magnets to decrease the expense of fabrication due to a reduction in the area and weight of the evacuated chamber used for scanning, to reduce the outgasing rate and the vacuum pumping requirements and to permit utilization of a horizontal mounting of a beam accelerator alongside a conveyor of material or objects to be irradiated thereby reducing ceiling height requirements and facility costs for high energy accelerators (above 0.5 MeV).
The present invention is generally characterized in a system for controlling the scanning of a beam of charged particles across a conveyor path to produce selective dose distributions including a particle accelerator for producing a beam of charged particles, a deflection magnet or a plurality of deflection magnets controllably operated to deflect the beam through a plurality of positions across the conveyor path, and control means coupled with the deflection magnet to sweep the beam through a plurality of positions across the conveyor path while maintaining the beam at each of the positions for a preselected time in accordance with the irradiation dosage required at each of the positions.
The present invention is further generally characterized in a method of scanning a beam of charged particles across a conveyor path to control the radiation dose at a plurality of positions along the line of scan including the steps of deflecting the beam incrementally from position to position along the line of scan, and controlling the length of time the beam remains at each position to control the radiation dose at each position.
Other objects and advantages of the present invention will become more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.